1. Field of the Invention
The present invention relates to transfer and separation technique in an electrophotographic image forming apparatus.
2. Description of Related Art
In the electrophotographic image forming process, the toner image is transferred to a transfer material which is the recording paper, using an electrostatic force.
The main transfer methods include: the corona transfer method in which the back surface of the transfer material is corona charged and then transferring is performed; and a transfer method in which the transfer material is pressed onto an image carrier using conveyor belt or a roller to which transfer voltage has been applied.
The latter method is advantageous in that, transfer properties are good because transferring is done by bringing the transfer material into close contact with the image carrier and there is little image disturbance at the time of transfer. Thus, this method is now being frequently used in high quality image formation apparatuses.
Also, the transfer method in which the transfer material is brought into close contact with the image carrier by the conveyor belt has the advantage of stable conveyance because the transfer material is conveyed by the conveyor belt.
Image formation apparatuses in which transfer is done by bringing the transfer material in close contact with the image carrier using a conveyor belt are disclosed in Unexamined Japanese Patent Application Publication No. 10-10885, Unexamined Japanese Patent Application Publication No. 11-38796, and Unexamined Japanese Patent Application Publication No. 9-160324.
In the inventions disclosed in these publications, various measures, such as changing the contact width of the transfer material with the image carrier, have been proposed for improving transfer properties.
After transfer, the transfer material is separated from the image carrier, but in transfer using a conveyor belt, a conductive conveyor belt is used and the transfer material is separated from the image carrier by electrostatic suctioning of the transfer material onto the conveyor belt to which transfer voltage is being applied.
In the high speed process, it became clear that there was a problem with the conveyance of the transfer material in the separation section in which the transfer material is separated from the image carrier.
This problem is described using FIG. 3 which is the figure for describing an embodiment of the present invention.
It is to be noted that the terms “upstream” and “downstream” in the present specification are used with reference to the conveyance direction of the transfer material.
The transfer of the toner image is carried out in a transfer region extending in the conveyance direction of the transfer material. The transfer region herein refers to the region where the transfer material is brought in close contact with the image carrier, and in FIG. 3 it is a rectangular region in which the transfer material conveyance direction from the point AP which is the upstream contact start point to the point BP which is the downstream separation point is the short side, while the width direction which is the direction at right angles to the transfer material conveyance direction is the long side.
The transfer region is formed by the transfer roller 70 and the nip regulating roller 74 that is disposed downstream with respect to the transfer roller 70.
The transfer material S separates from the image carrier at point BP which is the downstream end of the transfer region.
The conveyor belt 71 is bent at the point CP by the nip regulating roller 74, but the transfer material S is held and conveyed by the conveyor belt 71 in the downstream section from the point CP and is conveyed smoothly on the conveyor belt 71.
In the case where regular paper is used, the recording sheet S is conveyed smoothly on the conveyor belt 71 in the manner described above.
However, in the case where the transfer material S is thick paper, because rigidity is high, the transfer material S moves forward at point CP and separates from the conveyor belt 74.
As a result, the upper surface of the transfer material that carries the toner image comes in contact with the conveyance guide and problems such as image disturbance or conveyance defects occur.
In the image formation apparatuses disclosed in Unexamined Japanese Patent Application Publication No. 10-10885, Unexamined Japanese Patent Application Publication No. 11-38796, and Unexamined Japanese Patent Application Publication No. 9-160324, transfer property has been improved but the aforementioned problem with separation has not been solved.
The object of the present invention is to supply an image forming apparatus which: solves the aforementioned problem which occurs in the separation section in which the transfer material separates from the image carrier; performs favorable transfer for various types of transfer material; and carries out smooth transfer of the transfer material in the separation section.